The present invention relates to an etching method.
A gas containing a high concentration of fluorine atoms such as CHF3, CH2F2, CF4 or C4F8 is normally used as an etching gas in a dry etching process for forming grooves such as contact holes, in order to create a reaction seed containing fluorine through plasma discharge or the like. The etching process is implemented by using a processing gas achieved by mixing Ar which is mainly used for gas flow rate ratio control or O2 which is mainly used to improve penetration, i.e., to promote etching along the depthwise direction in such a fluorine-containing gas. Typical examples of processing gas combinations in the prior art include CHF3+Ar+O2, CH2F2+CF4+Ar, C4F8+CH2F2+Ar+O2.
CH2F2 is one of the gases most frequently used among the fluorine-containing gases mentioned above utilized in etching processes. The frequent use of CH2F2 is attributable to its high resist-relative selection ratio. It is to be noted that the term xe2x80x9cresist-relative selection ratioxe2x80x9d used in this specification refers to the value expressed as (average etching rate of the etching target film)/(etching rate of photoresist). Openings of grooves formed by using a processing gas with a low resist-relative selection ratio tend to be large and are not, therefore, desirable.
However, while the resist-relative selection ratio may be improved by raising the flow rate ratio of CH2F2 in the processing gas, there is a problem in that the shapes of grooves formed using such a processing gas become abnormal. This problem is assumed to be caused by H and HF that are generated when CH2F2 becomes dissociated during the plasma process and subsequently isotropically etch the areas around the holes. Accordingly, the flow rate ratio of CH2F2 is adjusted by using a processing gas having C4F8, Ar and O2 mixed with CH2F2 as described above in the prior art.
When C4F8 is used to control the flow rate ratio of CH2F2, the following problems occur. Firstly, while the shape of grooves that are formed is improved to a certain extent, deformation such as bowing, longitudinal streaking and waviness still occur and thus, a full improvement in correcting deformation is not realized. Secondly, the degree to which the resist-relative selection ratio becomes lowered far exceeds the extent to which the shape of the grooves is improved. In reality, the resist-relative selection ratio is lowered to approximately 3.0xcx9c4.0 and the openings of the grooves become wide.
A first object of the present invention, which has been completed by addressing the problems of the etching method in the prior art, is to provide a new and improved etching method through which the resist-relative selection ratio can be improved.
A second object of the present invention is to provide a new and improved etching method through which the etching shape can be improved.
In order to achieve the objects described above, the etching method according to the present invention, which is implemented on an SiO2 layer formed on a workpiece placed inside an airtight processing chamber by inducing a processing gas into the processing chamber is characterized in that the processing gas contains at least C5F8 and CH2F2 and in that the flow rate ratio of C5F8 and CH2F2 in the processing gas is essentially within a range of 1/4xe2x89xa6(C5F8 flow rate/CH2F2 flow rate)xe2x89xa61/2.
The processing gas may also contain O2.
In this case, it is most desirable to ensure that the flow rates of C5F8 CH2F2 and O2 in the processing gas essentially achieve a relationship expressed as ((C5F8 flow rate+CH2F2 flow rate)/O2 flow rate)=1.5/1, with the allowable range being 0.5/1xe2x89xa6((C5F8 flow rate+CH2F2 flow rate)/O2 flow rate)xe2x89xa63/1.
The processing gas may also contain Ar.
In addition, etching may be performed under a condition expressed as (SiO2 layer average etching rate/photoresist etching rate)xe2x89xa710, i.e. under the condition in which the resist-relative selection ratio is 10 or larger.